1. Field of the Invention
The present invention relates to an optical modulator used for optical communication and an optical multiplexing module using a plurality of such optical modulators. More particularly, it relates to a semiconductor optical modulator of the electroabsorptive type, and to an optical multiplexing module using a plurality of electroabsorptive modulators.
2. Description of the Related Art
A block diagram of a conventional optical multiplexing module having a multiplexing factor of two is shown in FIG. 6A. The optical multiplexing module 50 includes a combiner 1 that combines a pair of optical signals into an output optical signal LTt, a first modulator 2 that modulates a source optical signal LTs according to a first modulating voltage Vm1, a second modulator 3 that modulates the source optical signal LTs according to a second modulating voltage Vm2, a pair of attenuators 4, 5 that attenuate the optical power of the source optical signal LTs, and a splitter 6 that splits the source optical signal LTs into two signals for output to the two attenuators 4 and 5.
From the splitter 6, the source optical signal follows a path of optical length L1 through the first attenuator 4 and first modulator 2 to the combiner 1, and another path of optical length L2 through the second attenuator 5 and second modulator 3 to the combiner 1. The source signal comprises, for example, a regularly spaced series of narrow pulses of light, which are modulated in the modulators 2 and 3. Even though a source light pulse enters the two paths simultaneously, if path length L2 is longer than path length L1, the two modulated light pulses will reach the combiner 1 at different timings. Time-division multiplexing can be performed by setting the path length L2 so that the timing difference is equal to one-half the interval between successive output pulses from the first modulator 2.
The two modulators 2 and 3 ideally have identical insertion losses. In practice, however, fabrication inaccuracies may lead to different insertion losses.
FIG. 7 shows a perspective view of the structure of one example of a conventional semiconductor optical modulator. Formed as a semiconductor chip by the use of semiconductor fabrication technology, the semiconductor optical modulator 20 includes an electroabsorptive layer 21 that absorbs light to different degrees depending on an electric field intensity, a p-type clad layer 22 disposed above the electroabsorptive layer 21, an n-type clad layer 23 disposed below the electroabsorptive layer 21, polyimide layers 24, disposed on the right and left sides of the electroabsorptive layer 21, a contact layer 26 disposed above the p-type clad layer 22, and a pair of modulating voltage electrodes 27, 28 disposed respectively above the contact layer 26 and below the n-type clad layer 23.
If incident light LTin (the source optical signal LTs) enters the electroabsorptive layer 21 as indicated by the arrow in FIG. 7, and a modulating voltage Vm1 is applied to the modulating voltage electrode 27 while the lower electrode 28 is grounded, the electroabsorptive layer 21 absorbs the incident light LTin to different degrees, responsive to the modulating voltage Vm1; consequentially, a modulated light signal is output from the electroabsorptive layer 21. The electroabsorptive layer 21 has a limited light-absorbing capability per unit length, however, so to provide a modulation depth adequate for optical communication, the semiconductor optical modulator 20 must have at least a certain necessary length, this being the dimension of the electroabsorptive layer 21 in the direction of light propagation.
The modulators in general use are of the transmissive type, in which incident light enters at one end and is output from the other end. The necessary length dimension can be reduced by half, however, by using modulators of the reflective type, in which light enters and exits at the same end. In this case, the modulators are coupled to their respective light paths by optical circulators, not shown in the drawings.
FIG. 8A shows a typical example of the optical power level of the output optical signal LTt in the optical multiplexing module 50 in FIG. 6A when the first attenuator 4 and second attenuator 5 are not controlled, that is, when the power level of the output optical signal is not adjusted. The optical power level OPS2 of signals S2-1 and S2-2 from the second modulator 3 exceeds the optical power level OPS1 of signals S1-1 and S1-2 from the first modulator 2 by a value D. This power difference D may arise because of an unequal splitting ratio in the splitter 6, or because the absorption characteristics (extinction ratios) of the first modulator 2 and second modulator 3 differ, due to differing fabrication conditions.
In the conventional optical multiplexing module 50 in FIG. 6A, the first attenuator 4 and the second attenuator 5 adjust the power level of the source optical signal LTs so as to suppress this power difference D.
An optical multiplexing module is also known that adjusts the power levels of the output optical signals to a constant value OPSs by controlling the modulating voltages as shown in FIG. 6B, instead of by using attenuators. In place of the attenuators 4 and 5 in FIG. 6A, the optical multiplexing module 51 in FIG. 6B has a first modulating voltage controller 7 that biases the modulating voltage Vm1 input to the first modulator 2, and a second modulating voltage controller 8 that biases the modulating voltage Vm2 input to the second modulator 3, thereby adjusting the power levels of the optical signals output from the first modulator 2 and the second modulator 3 to a uniform level OPSs.
Due to differing fabrication conditions or an insufficiently accurate fabrication process, however, the two modulators 2, 3 may have different extinction ratio characteristics, and thus respond differently to biasing of the modulating voltage, making a uniform output level difficult to achieve. For example, one modulator may operate in a linear region of its extinction ratio characteristic, while the other modulator operates in a nonlinear region. This is particularly apt to occur when modulators of the reflective type mentioned above are used, since their extinction ratio characteristics tend to show more non-linearity than is seen in modulators of the transmissive type.
FIG. 9 shows examples of the extinction ratio characteristics of electroabsorptive modulators of the reflective type (solid line) and transmissive type (dotted line). If a reflective modulator having the extinction ratio characteristic indicated by the solid line in FIG. 9 is employed, and if the modulating voltage is biased at minus one volt (xe2x88x921 V) with an amplitude of xc2x11 V, then the excellent linearity of the extinction ratio characteristic from 0 V to xe2x88x922 V can be used. If the bias voltage is set to xe2x88x922 V, however, then the amplitude of the modulating voltage extends into the region of poorer linearity below xe2x88x922 V. Moreover, even if the bias voltage is set to xe2x88x921 V, if the amplitude of the modulating voltage exceeds xc2x11 V, then the region of poor linearity in the extinction ratio characteristic below xe2x88x922 V must be used.
If the optical multiplexing module 50 in FIG. 6A uses semiconductor optical modulators 20 having the imperfectly linear extinction ratio characteristics described above, then even though the optical power levels of the optical signals output from the modulators are adjusted to a constant value OPSs by the use of attenuators, fabrication variations and the non-linearity of the extinction ratio characteristics of the semiconductor optical modulators may cause the modulating fields generated by the modulating voltages Vm1 and Vm2 to have varying effects, so that the modulated output signals vary as shown in FIG. 8B, leading to transmission quality problems such as an inadequate modulation depth.
If the optical multiplexing module 51 in FIG. 6B uses semiconductor modulators having imperfectly linear extinction ratio characteristics, then it may be impossible to adjust the power levels of the optical signals output from the two modulators 2 and 3 to a constant value OPSs without bringing at least one of the modulating voltages Vm1 and Vm2 into the nonlinear region of the extinction ratio characteristic of the corresponding modulator, so that, as shown in FIG. 8B, identical and adequate modulation depths are not obtained.
An object of the present invention is to provide an optical multiplexing module having a uniform modulated output level despite fabrication variations and imperfectly linear extinction ratio characteristics.
Another object of the invention is to provide an optical multiplexing module having a simplified structure.
Yet another object of the invention is to provide a semiconductor optical modulator for use in such an optical multiplexing module.
To achieve the last of these objects, the invention provides a semiconductor optical modulator having an electroabsorptive layer, a pair of modulating voltage electrodes, and a direct-current voltage electrode. The electroabsorptive layer absorbs incident light responsive to an electric field generated by a modulating voltage applied to the modulating voltage electrodes. The electroabsorptive layer also absorbs incident light responsive to an electric field generated by a direct-current voltage applied to the direct-current voltage electrode. The direct-current voltage can be adjusted to set the optical power level of the modulated output light signal to a desired value. For example, the direct-current voltage can be increased until the optical power level is reduced to a predetermined target level.
Since the output optical power level of the invented semiconductor optical modulator is controlled by direct control of absorption in the electroabsorptive layer, accurate absorption control is possible despite fabrication variations.
The direct-current voltage electrode may be disposed so that it is positionally in series with one of the modulating voltage electrodes in the direction of light propagation. The output power level of the semiconductor optical modulator can then be controlled without shifting the modulating voltage away from the optimal or most linear part of the extinction ratio characteristic.
The semiconductor optical modulator may also have a reflective film on one facet that reflects light back to an incident facet, so that the light traverses the electroabsorptive layer twice.
The invention further provides an optical multiplexing module having a plurality of semiconductor optical modulators as described above, and at least one direct-current voltage controller for adjusting the direct-current voltage supplied to the direct-current voltage electrode of at least one of the semiconductor optical modulators, so as to equalize the optical power levels of light output from the plurality of semiconductor optical modulators. Preferably, one direct-current voltage controller is provided for each semiconductor optical modulator. In this case, the optical power level of the multiplexed signal output from the optical multiplexing module can be controlled by first applying modulating voltages without applying direct-current voltages, detecting the optical power levels of the light signals output from the individual semiconductor optical modulators, setting a target optical power level equal to or less than the minimum detected optical power level, and then controlling the direct-current voltages supplied to the semiconductor optical modulators so that light is output from each semiconductor optical modulator at the target optical power level.
The structure of the invented optical multiplexing module is simplified because it is not necessary to use external attenuators or bias the modulating voltage signals, and a uniform optical power level can be obtained even with optical modulators, such as reflective optical modulators, having significantly nonlinear extinction ratio characteristics.